An image forming apparatus such as a tandem type color copying machine using an electrophotographic process is provided with image forming sections, each configured with a photoconductive drum, a charging device, a scanning optical device and a developing device, respectively for yellow (Y), magenta (M), cyan (C) and black (K) colors along an endless intermediate transfer belt. The image forming apparatus is configured such that a toner image of each color Y, M, C and K is successively superimposed on the rotating intermediate transfer belt to form a color image, thereafter, the color image is transferred form the intermediate transfer belt to a recording sheet to be fixed and outputted.
Also, there is an image forming apparatus wherein a rotating endless second transfer belt to be in pressure contact with the intermediate transfer belt is disposed to face the intermediate transfer belt, and a toner image is transferred from the intermediate transfer belt to the recording sheet by passing the sheet between the intermediate transfer belt and the second transfer belt while pressing the recording sheet (for example, refer to Patent Document 1: unexamined Japanese patent application publication No. 2007-11107).
If bias or meandering of the intermediate transfer belt or the second transfer belt occurs while running, color shift occurs, resulting in deterioration of image quality. Therefore, a control to detect and correct the bias of the belt is popularly performed (Patent Document 1: unexamined Japanese patent application publication No. 2007-11107). For example, in Patent Document 1, bias correction is performed by swinging rollers on which the belt is installed while maintaining the distances between a plurality of the rollers (Refer to unexamined Japanese patent application publication No. 2007-11107). Also, in an image forming apparatus provided with an intermediate transfer belt and a second transfer belt, the bias correction is performed respectively for the intermediate transfer belt and the second transfer belt.
The intermediate transfer belt and the second transfer belt are configured to be changed between a pressure contact sate and a separate state, and for example, they contact one another with pressure only in a necessary period of image forming. Each belt runs independently in the separate state, however in the pressure contact state, one belt is affected by a running condition and the bias correction of the other belt. Therefore, if correction control is performed uniformly regardless of the pressure contact state or the separate state, there has been a problem that appropriate bias correction is difficult for both pressure contact state and separation state.